1. Field of Invention
This invention relates to keyboard switches or push button type transducers for use in electronic circuits in which the key, when depressed, moves an elastomeric conducting element into contact with or into proximity with two conductive regions on a fixed member.
2. Prior Art
In keyboard switches of this type, the key normally carries or has associated therewith a striker member which when depressed by the key moves toward the conductive elements on the fixed member, usually a printed circuit board. The striker member may carry either a dielectric sheet backed by a conductive sheet, an elastomeric conductive element, or a resistive element generally of rigid construction and backed by a compressible mounting. Such transducers may provide a pure capacitive coupling between the conductive elements on the fixed member or a combination of a resistive capacitive coupling, or a pair of resistive couplings.
An example of the pure capacitive type transducer is shown in the patent to Frank A. Walker Jr., et al, U.S. Pat. No. 3,965,399, dated June 22, 1976 and entitled PUSH BUTTON CAPACITIVE TRANSDUCER. This patent teaches a special mounting of the striker having a deformable dielectric sheet and a conductive sheet thereon to insure uniform contact with the conductive elements for a uniform capacitive coupling upon operation. However, the physical size of the striker in a keyboard application is such as to limit the amount of capacitance developed, and the build up of a static charge with plunger operation requires more sophisticated electronic circuitry to separate valid switch closure signals from signals generated by the static charge and stray pick up signals between switches.
The patents to S. Zilkha U.S. Pat. No. 3,797,630, dated Mar. 19, 1974 and entitled KEYBOARD FOR ELECTRONIC CIRCUIT, and Peter Pointon et al U.S. Pat. No. 3,951,250, dated Apr. 20, 1976, and entitled CAPACITIVE KEY FOR ELECTRONIC CIRCUIT both show arrangements in which the striker member carries an elastomeric conductive member to contact conductive regions on a fixed member such as a printed circuit board. When the elastomer pad contacts the plates it closes the circuit and forms two capacitances with corresponding plates on the other side of the printed circuit board, the board itself acting as the dielectric. These two capacitances are connected in series thru the resistive path formed by the elastomeric conductor. In the Zilkha patent, the effects of static charge buildup on the elastomeric conductive striker can, in certain circumstances, cause erroneous operation. The Pointon et al patent shows various modifications of this keyboard switch in which the dielectric sheet over the fixed conductive elements has a conductive sheet or conductive regions positioned on the dielectric sheet and opposite the fixed conductive elements on the printed circuit board to be contacted by the striker. This arrangement is designed to reduce the electrostatic charge on the capacitive coupling between the conductive elements. The Pointon et al patent shows variations of this switch design in which the striker member carries a ceramic with dielectric therein to contact fixed conductive elements on the printed circuit board and it also shows a striker having a solid surface of resistive material backed by a foam mounting and contacting the dielectric on the conductive elements positioned on the printed circuit board. In the Pointon et al teaching, where the elastomeric conductive striker is used, the dielectric between the conductive elements or conductive areas is in itself a printed circuit board. The conductive elastomer bridges the metallic pads or conductors making a resistive coupling and this combination transfers the key closure signal that is obtained capacitively from a third plate on the opposite side of the printed circuit board. With this method, very small values of capacitive coupling is obtained. While this structure has the advantage of dissipating static charge that tends to be generated on the plunger or striker as it moves up and down, it results in a complex switch design and requires more sophisticated drive and detection circuitry.